This invention relates to a thickener containing starch, the aqueous sol of which is characterized by its superior low-temperature stability as indicated by its resistance to repeated freezing and thawing cycles, and to foodstuffs containing same.
Starch, in general, contains two types of polymers: a linear one known as amylose and a branched polymer known as amylopectin. Waxy starches, produced from such grains as waxy maize, waxy rice and waxy sorghum, contain a very much higher level of amylopectin than ordinary corn starch, and are of particular value in applications where the starch serves primarily as a thickener or stabilizer, and where a stable sol which resists retrogradation is desired.
To meet the critical needs of the food industry, in terms of food quality, processing, packaging and distribution, native waxy starches are often modified by numerous techniques known in the industry to change the behavioral characteristics yet still essentially retain the caloric value of the native (unmodified) starch.
One type of modification commonly employed in the art is crosslinking of the starch. When an aqueous dispersion of native starch is heated, the starch granules begin to swell, and the dispersion develops a short, salve-like texture which is essential in imparting palatability and in thickening food systems. However, during the process of cooking native starches, this textural state, particularly with waxy starches, rapidly changes to an elastic, rubbery state when the swollen granules rupture, and minor variations in cooking time, temperature, and concentration as well as shear and pH are sufficient to effect this transformation. Crosslinking modifications act to strengthen the granules by reinforcing the hydrogen bonds which are responsible for holding the granules intact and thus are used to overcome the extreme sensitivity of the swollen starch granules to handling and processing conditions.
Aqueous dispersions of crosslinked starch are often used under conditions which involve prolonged storage at relatively low temperatures and/or exposure, at times, to repeated freezing and thawing cycles. Thus, starch dispersions are used in fruit pie fillings, which are frequently canned, as well as in a number of frozen foods, such as frozen pies, soups and the like. In the case of canned food products, these are often stored in warehouses which have no heating facilities and may, therefore, be at very low temperatures for prolonged periods. Frozen foods are also subject to long term storage at very low temperatures. Under such circumstances involving exposure to low temperature, there is a distinct loss in the hydrating power of the starch which is present in such food products, thereby resulting in syneresis, an exudation of liquid, together with a marked deterioration in the texture, color and clarity of the food product. While sols of waxy starches are superior in stability to those of regular starches, even they are prone to intermolecular association during storage at or near freezing temperature.
Attempts to overcome these difficulties have in the past involved the introduction of substituted branches onto the starch molecule by means of various chemical derivatization reactions, for example, reacting the starch with a monofunctional reagent to introduce substituents such as hydroxypropyl, phosphate, acetate or succinate groups. Such substituents stabilize the starch by interfering with the association between molecules or portions of the same molecule, thus reducing the tendency of the substituted starches to lose their hydration ability and clarity on storage particularly at low temperatures.
These derivatization reactions alone may be carried out on native starches to improve their low-temperature stability, but are frequently employed in combination with crosslinking to obtain starches for use as thickeners in canned pie fillings, retorted puddings, etc., which will keep the food from losing its clarity and texture during storage or shipment in the winter and while frozen.
In recent years there has been a trend toward searching for starches which have all of the properties of a modified starch but without the chemical treatment. U.S. Pat. No. 3,525,672 issued Aug. 25, 1970 to Wurzburg et al. discloses treating an inhibited starch with an enzyme such as beta-amylase to impart freeze-thaw stability thereto, but this is not at present a commercially feasible process.
Accordingly, it is an object of the present invention to provide a thickener containing starch the sol of which exhibits the low-temperature stability characteristics of chemically stabilized waxy starches.
It is a further object to provide a thickener containing a starch particularly suited for use in preparing foodstuffs having good low-temperature stability.